Location-aware technologies compute the location of an object. These systems differ in terms of accuracy, coverage, cost of installation, and maintenance. Global Positioning Systems (GPSs) use satellite signals and work in outdoor environments. GPS systems, however, require direct line of sight and do not work in an indoor environment.
Systems have been developed in the past that use the strength of wireless access point beacon signals in an outdoor environment to calculate the position of a mobile user. The active badge system uses infrared (IR) technology but it has several limitations such as limited range of IR, direct line-of-sight requirements for IR, and the high-cost of installation of a dedicated system just for object tracking. Another early indoor object tracking system was the Active Bat system. This system used time-of-flight of ultrasound transmissions between ultrasound transmitters that are attached to the object and ceiling mounted receivers. Again, this system was a dedicated system dedicated to object and people tracking and was not commercially successful because it required installation of a new infrastructure.
One technique developed was to create a database of wireless beacons and use that information together with the signals received by a mobile to compute the location of the mobile. Other techniques use Radio Frequency (RF) wireless signal strength information and triangulation to locate objects in an indoor environment. However, these methods provide poor indoor positioning accuracy because RF signal amplitude is greatly affected by metal objects, reflective surfaces, multipath, dead-spots, noise and interference.
Other methods use time of arrival information of cellular RF signals and cell tower (i.e., base stations of licensed wireless systems) triangulation to determine a coarse radio-based position and then use that to assist a GPS system to determine the GPS-based position. Cell tower triangulation is limited in accuracy and reliability because of the coarse number of cell towers and multipath issues. Also, these methods require the presence of a GPS signal and a GPS time reference for measuring time of arrival of cellular RF signals. Their approach does not work in indoor situations where GPS signals are weak or not present. These methods also require time synchronization and prior knowledge of the position of cellular base-stations. Other methods have also used cellular tower triangulation to determine a coarse pre-fix position and use that to assist a GPS system determine the GPS-based position.